This authoritative volume presents a detailed, documented history of the United States Atomic Energy Commission (AEC) from 1947 to 1952. It begins with the assumption of responsibility for the nation's nuclear energy programs by the Commission and concludes with the detonation of the first thermonuclear device. That period includes the Marshall Plan, the Berlin Blockade, and the Korean War. The authors use a chronological narrative to describe the realization that the Commission's initial idealistic goals of developing peaceful uses of the atom would have to accommodate the military applications demanded for national security. During that period the Commission presided over the conversion of the remnants of the Manhattan Project to a larger complex of government and private installations with thousands of employees engaged in developing nuclear power for military and civilian applications. The work combined with the first volume, The New World , and the third volume, Atoms for Peace and War 1953-1961, provides comprehensive coverage of the development of nuclear energy in the United States from 1939 to 1961.